1. Field of the Invention
This invention relates to an electric circuit device and a substrate used therefor, and more particularly to an electric circuit device and a substrate used therefor functioning as a noise filter with a wide frequency coverage and excellent high-frequency characteristics.
2. Description of Related Art
Recently, digital circuit technology such as LSI (Large Scale Integrated) circuit technology is adopted in not only computers and communication-related equipment but also consumer electronics and in-vehicle equipment.
A high-frequency current produced in the LSI circuit or the like does not stay in the vicinity of the LSI circuit but flows to the wide area of a component-mounted circuit substrate such as a printed-circuit substrate. The high-frequency current then inductively couples to signal wires and grounding wires and leaks as an electromagnetic wave from signal cables or the like.
In mixed-signal circuits in which analog circuitry and digital circuitry are combined, for example, a circuit in which a part of a conventional analog circuit is replaced with a digital circuit, and a digital circuit having analog input/output, one of the serious problems is electromagnetic interference from the digital circuit to the analog circuit.
An effective solution of this problem is to separate the LSI circuit, which is a source of the high-frequency current, from a power supplying system with respect to the high frequency, that is to say a “power decoupling” technique. Known as a noise filter employing the power decoupling technique is a transmission-line type noise filter (e.g. Japanese unexamined patent publication No. 2004-80773).
This transmission-line type noise filter comprises a first electrical conductor, a second electrical conductor, a dielectric layer, a first anode and a second anode. Each first and second electrical conductors is in the form of a plate. The dielectric layer is disposed between the first and second electrical conductors.
The first anode is connected to one end of the first electrical conductor in a longitudinal direction, while the second anode is connected to the other end of the first electrical conductor in the longitudinal direction. The second electrical conductor functions as a cathode to connect to reference potential. The first electrical conductor, dielectric layer, and second electrical conductor constitute a capacitor. The thickness of the first electrical conductor is so set as to substantially prevent the temperature rise caused by a DC (direct current) component of the current flowing through the first electrical conductor.
The transmission-line type noise filter is connected between a power source and an LSI circuit so as to feed a current from the power source through a path made up of the first anode, the first electrical conductor and the second anode to the LSI circuit, while attenuating an AC (alternating current) current produced in the LSI circuit.
As discussed above, the transmission-line type noise filter has a structure of a capacitor, and uses the first and second electrical conductors, which are two electrodes of the capacitor, as transmission lines.